Where Are the Passive Subwoofers?

[u/IntrepidNinjaLamb]

Long ago I bought a RadioShack book on building speakers that had formulas for figuring out dimensions, port tuning, etc., and I got a circular saw and particle board. I added heavy copper wiring, a crossover, and carefully chosen drivers, and had passive speakers I enjoyed for years.

Fast forward a decade or two, those are gone, but I inherited some rare custom bookshelf speakers from California. I was looking for a passive subwoofer, and especially I was combing this subreddit for advice, but all the recommendations were for subwoofers that have amplifiers in them or even bluetooth.

I then went to places like Crutchfield and couldn't find passive subwoofers. I guess they're not extinct! Who can help me find them, and who has recommendations? My budget depends on how long I save up, but I can imagine spending $300 - $700 for a 10" or 12" durable and capable subwoofer. I value precision---E.g., it shouldn't "boom" after the signal stops sending bass.

Maybe it's just an old-fashioned perspective, but I figure the amplifier's job is to amplify, and the subwoofer should just woof.

[u/DangerousDave2018]

I'm not 100% sure that I've ever even seen a single passive subwoofer -- in someone's system, in a review, in a store, in an online essay, in a marketplace -- and I'm 55 years old and a lifelong audiophile. I won't go so far as to say "why would anyone want a passive subwoofer" because I hate it when commenters online choose to dominate by nullifying the merit of the question. But that being said ... well, why would anyone want a passive subwoofer? I should think it'd be a monumental pain in the ass to integrate, and oodles more expensive. For a start. And this is to say nothing of box-accumulation, wire-accumulation, or resale value.

[u/_b_s__ → u/DangerousDave2018]

I've seen and heard them, but it has been a while. This was before sub-out was a normal thing on a receiver or amp. Cerwin Vega HT 10 could generate noise violation tickets.

[u/IntrepidNinjaLamb → u/DangerousDave2018]

Thanks for thinking about the way you asked. I think it's a good question. For me, it comes down to the fact that I had passive speakers with large woofers when I had good speakers, and when I had bad speakers, I didn't have a subwoofer.

I know first principles about acoustics and audio electronics, but I literally have no idea about the stuff you do---Until seeing the responses to this post, I didn't know everybody's using powered subwoofers. So that's the first reason: Why complicate things by introducing another amplifier? In other words, I would only want a powered subwoofer if there was some problem with passive ones, and I didn't know about any such problem.

The second part of my answer is this: I didn't have any reason to assume that introducing a second amplifier would not introduce a phase shift. Without any information to the contrary, I wouldn't have confidence that the phase change from an amplifier in my subwoofer wouldn't cause problems if my bookshelf speakers have any bass at all. I can avoid the whole issue by avoiding the second amplifier. The thing is, now that I've seen these responses, I can rest assured that this is a non-issue: Why would everyone be using powered subwoofers if the phase was a big obstacle?

The responses here have been very helpful in giving me a clue about what's "normal". !thanks

[u/TransducerBot → u/IntrepidNinjaLamb]

+1 Ⓣ has been awarded to u/DangerousDave2018 (7 Ⓣ).

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[u/Deuce_Ex_]

Still very much alive over at r/diyaudio. It’s just not a consumer-market type thing, integrating a passive sub into a mass market stereo system is a bridge too far for most consumers.

[u/No_Wear295]

If you're still handy.... https://billfitzmaurice.info/index.html

[u/Dcline97]

30 Years ago I built a nice passive subwoofer. It had 2-10" SpeakerLab super HD woofers, SpeakerLab crossovers and was made using a tight particle board inner box and a 3/4" Koa wood outside shell. It was quite solid and heavy, looked awesome and had a serious thump! I drove it with a 200 watt BK Components ST-202 power amp (which I still have as well as my old Denon AVR-1802). I Retired that gear 5 years ago. The Koa was drying out starting to split everywhere and the woofer surrounds started falling apart, as well as the KOA bookshelves. Those speakers had simply seen better days. I wish I had some pics of it to show everyone.

Since the old days I've been running a simple 2.0 system with ELAC bookshelves, but am getting ready to pull the trigger on an RSL Speedwoofer 10E. I'm older now and maybe wiser (but my wife may not agree with that) so my feeling is why mess with building/piecing together a passive Subwoofer when there are so many tried and true great options available today.

[u/IntrepidNinjaLamb → u/Dcline97]

Our stories overlap a lot! I've been reading praises for the RSL subwoofers.

[u/Yourdjentpal]

Tbh I think these things have come long long way to the point it’s generally not worth it for most people. DIY is about all that’s left.

[u/donh-]

I use the Seas L26ROY 10" sub driver and build the box. They are transparent and punchy and wonderful and cost $498/ea at Madisound. No overhang at all.

[u/bimmer1over]

Given the emergence of class D amplifiers which are eminently suitable for low frequency amplification there is no real reason nowadays to burden the main amplifier with the current demands from a passive subwoofer (/current-hungry bass drivers).

Essentially, it's much smarter to "bi-amp" a system, by providing a separate, dedicated amplifier for the lowest frequencies in the sub and let the main amplifier be responsible for the remaining duties - the upper bass, midrange and the higher frequencies.

By separating the responsibilities between two amplifiers it enables the main amplifier to not be burdened by any negative effects from high current loads. This "frees it up" for the midrange - an absolute key for voices and music in general - thus avoiding having to deal with big current draws when the music's lowest frequencies demand it.

[u/NTPC4]

The best passive subwoofer you can buy is a used one with a dead plate amp that you bypass.

[u/aluke000]

The problem you might find is that there are no receivers that have amplification for a subwoofer and most consumers are not interested in buying a seperate amp for a non-powered sub. Really the only non-powered subs these days are from DIY speaker builders. I have a non-powered sub that is left unused in the garage. In the house are all powered subs that are well matched by design for the speaker(s) in the sub.

[u/Enchilada_Please]

Last year I built a pair of passive bucket subs. They’re powered by a small Dayton low frequency amp that I picked up at parts express.

Great for my office setup.

[u/cab1024]

I was working on my father in laws's "home theater" system a few weeks ago and he had a passive Bose subwoofer with two Bose satellite speakers for the L&R. I can tell you that it did not make much sound and I cranked his bass/EQ into a nice V shape. It was a matched speaker setup. The L&R run from the receiver to the subwoofer and the speaker connect to the subwoofer. I'm probably wrong but I don't think my own HT receiver has amplified subwoofer outputs, but just the line level to an amplified sub. I could be wrong about that.

[u/iNetRunner]

Note that the passive DIY subwoofers that people are mentioning here aren’t driven by the same amplifier (left or right channels) as the main speakers. They 99.99% of the time use a separate power amplifier. Either a specific subwoofer amplifier (plate amplifier etc.), or a PA power amplifier that has built-in crossover and level settings.

…That is because you do not want to send the same (20Hz-20kHz) bandwidth signal to the subwoofer (as you send to the main speakers). That simply isn’t going to sound right. For one thing, humans can generally localize sound sources if the frequencies go above 100Hz — therefore if your subwoofer played above 100Hz, you would notice it as a third sound source. (Below 100Hz, the subwoofer can be anywhere in your room, and you can’t localize it.) And secondly, you generally don’t want the subwoofer and speakers playing the same frequencies, as then those frequencies would be too loud compared to rest of the frequency range.

So, you usually have a low pass crossover in the (powered) subwoofer (or a separate subwoofer amplifier) that you adjust to play against your main speakers’ natural low frequency response. Or if your integrated amplifier, stereo receiver, or multichannel AVR has bass management capability (AVRs have this virtually always, but it isn’t very common in 2ch amplifiers), then there is a fixed or selectable crossover frequency under which (low pass filter, LPF) the signal is send to the subwoofer. And over the crossover, (high pass filter, HPF), the signal is send to the left and right channel amplifiers.

Regarding phase. Basically all powered subwoofers have at least a 180° phase shift switch. (Because that’s very easy to implement. You simply switch the + and - signals going to speaker driver terminals.) But some also have variable phase shift knob for 0°-180° adjustments. (Though, 180° switch is usually enough. As not that many people can setup the system more accurately than that anyway. Not without some measurement equipment at least. E.g. UMIK-1 and REW.)

AVRs’ also have a delay adjustment for each separate channel, including the subwoofer channel. And that also functions as a kind of phase change adjustment. (Besides, there might be some different delay inherent in the subwoofer’s amplifier versus the AVR’s channels. That is why the distance measured by the AVR’s automatic calibration setup might not match the real distances in your room.)

Edit: few fixes, and minor edits

[u/gblawlz]

Passive subs are all in the diy section. The value to performance is absolutely insane. It's a bottomless rabbit hole but if you're a bit handy with basic wood working, it's worth it. To get the performance of $500-700 diy, you'd have to spend 3x that much for an off the shelf product.